A process for forming inner diameter finned tubes was developed for fabricating tubing. The tubes were produced by cold working the tube with a tube reducer over a tapered grooved mandrel directly to finished size. The mandrels for the process were specially fabricated of maraging steel at a cost of approximately $3,000 U.S. a piece. One mandrel would produce an average of only three to four units of 50 ft. (15.2 m) tubes of INCOLOY.RTM. alloy 800HT.RTM. on a half ring die tube reducer, such as a McKay.RTM. tube reducer. As the high-strength corrosion resistant alloy was forced over the mandrel, raised portions of the mandrel would eventually be rendered inoperable by galling or cold welding. Extensive galling or cold welding of the mandrel caused the product to have unacceptable interior fins. For this reason, mandrels were required to be replaced after producing an average of only 3 to 4 tubes when using a half ring die tube reducer. A full ring die, such as a tube reducer produced by Wayne, provides less mandrel wear, producing about forty to sixty 50 ft. (15.2 m) tubes per mandrel. The problem with the full ring die is the higher degree of difficulty in machining mandrels and the resulting higher mandrel price than for mandrels for the half ring die design. In addition, full ring die design tube reducers are significantly more expensive machines to purchase than half ring die design tube reducers.
When using a short stroke, half ring die tube reducer, an enormous force is required to cold work alloys containing at least 30 wt % nickel and 10 wt % chromium (percentages of alloy components are given in weight percent). Cold working is especially difficult with internally finned tubes having outer diameters of less than about 4 in. (10.2 cm). Internally finned tubing of this small diameter are especially difficult to form due to the large forces against a relatively small diameter mandrel.
It is an object of this invention to provide a method of producing internally finned tubing in a manner which extends the useful tool life of a mandrel.